A prepreg has been used as an interlayer insulating material for a printed wiring board for use in the field of electric and electronic industries. The prepreg includes various products including a paper-phenol prepreg in which paper, a backbone material, is impregnated with a phenolic resin, a glass-epoxy prepreg in which a glass cloth, a backbone material, is impregnated with an epoxy resin, a glass-polyimide prepreg in which the glass cloth, a backbone material, is impregnated with a polyimide resin. In recent years, in accordance with the trend of a thinner layer thickness for the printed wiring board, there has also been a growing demand for the prepreg, which is a component for the interlayer insulating layer of the printed wiring board, having a thinner thickness as well as high reliability.
For the method for producing the prepreg, each producer adopts a production method having distinctive features. Referring to a basic configuration of a general prepreg production device without accessory equipment such as an accumulator, the production method as shown in FIG. 5 will be the one adopted most widely, in which a resin composition for impregnating the backbone material is produced as vanish using a vanish reaction vessel 17 with a formulation having various characteristics. The vanish is sent to a circulation tank 18, from where the vanish is sent to an impregnation vat 20 in a step for impregnating the backbone material with the resin for circulation.
The step for impregnating the backbone material with the resin comprises means for axially bearing the backbone material roll and for continuously unwinding the backbone material 5. The backbone material 5 unwound therefrom is typically sent through a pre-impregnation vat 19 to the impregnation vat 20, where the backbone material 5 is subjected to resin impregnation either by dipping or by kiss-coating. After the impregnation vat 20, the impregnated backbone material is run through a vertically arranged drying tower 21 adopting a heating method such as hot air circulation or heat radiation for drying the impregnated resin to a half-cured state (B-stage) and is finally cooled to be wound and collected as a prepreg roll 22.
The prepreg produced in this manner has been widely accepted into a market when a backbone material having a weave texture such as a glass cloth is used, because the prepreg having a thickness of 30 μm can be produced by use of the cloth having a thickness of about 20 μm, although there are some problems such as a crease of the cloth and the like.
The use of a prepreg employing a backbone material having a weave texture such as a glass cloth, however, has caused a problem when forming a via hole in which carbon dioxide laser hole-making is required after it is processed to a copper clad laminate. More specifically, the hole-making of the copper clad laminate with a carbon dioxide laser causes a bad shape to an inner wall of the via hole after the hole-making due to inferior workability of the glass cloth in an interlayer insulating layer.
Non-woven fabric type backbone materials, such as a glass non-woven fabric, an aramid non-woven fabric and the like, have recently been used instead of cloth-type backbone materials in order to solve such a problem. Certainly, the shape of the inner wall of the via hole or the like formed with the carbon dioxide laser has been remarkably improved by replacing the backbone material with a non-woven fabric, and thus great progress has been achieved in the art.
A non-woven fabric is not like a cloth-type which is alternately woven by warps and wefts, but like a felt, it is made to a sheet by pressing a glass fiber, an aramid fiber or the like. Consequently, the strength of the non-woven fabric backbone material itself is lower, and the resistance to external stress, such as tensile stress, is smaller, compared with the cloth-type backbone material.
When drying the resin impregnated non-woven fabric with the method employing the above described vertical drying tower, the non-woven fabric with a required quantity of resin impregnated has to run through the drying tower under the load of the weight of the impregnated resin. The thinner the non-woven fabric is, more frequently it tends to break in the drying tower before the impregnated resin becomes a half-cured state to cause to suspend the process, thereby remarkably reducing a production yield. Such a problem tends to occur very frequently when the non-woven fabric for use as a backbone material has a nominal thickness of 70 μm or less, but the use of the non-woven fabric having a nominal thickness of 30 μm or less as a backbone material has been regarded as almost impossible.
Even when using a woven fabric such as a glass cloth as the backbone material, the above described conventional resin impregnation method naturally cannot secure safety of the process due to a tendency of rupture in the vertical drying tower, if the woven fabric having a thickness of 20 μm or less is to be impregnated and dried. A woven fabric may tend to break less likely than a non-woven fabric, but a thin woven fabric is not highly reliable as compared with a thick woven fabric.
From the above, the establishment of a production method capable of stably producing the prepreg having an unprecedented thickness by using a non-woven or woven fabric as the backbone material has been awaited in the market, as the prepreg capable of securing the above described carbon dioxide laser hole-making property.